Perpendicular magnetic recording (PMR) is important for the future of the magnetic recording industry because it offers higher areal density than the current longitudinal magnetic recording (LMR). This is due to the fact that the P medium is thermally more stable than that used for LMR. At present, LMR has achieved over 100 Gigabits per square inch (Gbpsi) in the laboratory and more than 60 Gpsi in products currently offered at the market place. In order to further extend the LMR recording density, two main obstacles have to be overcome. The first one is the thermal stability of the LMR recording media which arises because its thickness has to decrease to the extent that thermal energy could randomize the recorded bits. The second one is the ongoing increase in the write field needed to record on the high coercivity LMR media.
This high coercivity is needed to achieve high bit resolution and good thermal stability. Both obstacles to LMR could be considerably lowered if PMR were deployed instead. Thicker PMR media with a magnetically soft under-layer film (SUL), could be used to alleviate the thermal stability problem. A PMR writer provides a larger write field than that of LMR, which is limited to the fringe field from its write gap.
An example of a perpendicular writer of the prior art is shown in FIG. 1. Magnetic yoke 11 is surrounded by field coil 12 and includes main pole 13 that terminates as a write pole tip at the recording surface. Return pole 14 conveys the magnetic flux generated by coil 12 down to within a distance S of the recording surface 16 while downstream shield 15, running parallel to the recording surface, completes the magnetic circuit with the exception of gap g1 into which some of the write field is diverted. The main flux passes through recording layer 17, into SUL 16, and then back up into downstream shield 15 on the far side of g1.
One of the problems for a PMR writer of the type seen in FIG. 1 is the extent to which its write field spreads to its surroundings, thereby jeopardizing the stability of the recording bits. One approach to dealing with this has been the approach described above namely the introduction of a downstream shield (15 in FIG. 1) that is separated by a small gap (g1) from the PMR write pole so as to minimize the amount of flux returned from the PMR media. However, we have found that this solution to the wide spreading fringe fields problem is not quite adequate.
As shown in the attached simulation plots:—FIG. 2a shows the field plots for a single pole writer without a downstream shield while FIG. 2b shows the field plots for a single pole writer with a downstream shield, with the left side of the graph relating to the downstream end (i.e. the mirror reflection of FIG. 1). It is clear that the downstream shield has indeed reduced side fringing in the downstream fields, but there is still a large side fringing field spread on the upstream side, affecting the adjacent track recording bits. In this case, there could be as much as 20% of the maximum write field applied to the adjacent upstream tracks which could cause its randomization after repeated writing in the presence of thermal disturbance. Such a situation is obviously very undesirable for high track density recording.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 4,656,546 (Mallory) describe a downstream shield. In U.S. Pat. No. 5,003,423, Imamura et al disclose pole shields on the sides, upstream, and downstream. Das, in U.S. Pat. No. 5,075,956, shows a magnetic pole with side shields. Gill et al in U.S. Pat. No. 5,621,592 and also in U.S. Pat. No. 5,515,221, teach laminated shield layers.